1. Field of the Invention
The present invention relates to methods and apparatuses for fabricating quantum dot functional structures, quantum dot functional structures, and optically functioning devices. More particularly, the present invention relates to a method and an apparatus for fabricating a quantum dot functional structure, a quantum dot functional structure, and an optically functioning device, which provide the following outstanding features. The features make it possible to control the diameter of and alleviate the contamination of ultra-fine particles that are expected to provide various functions resulting from the quantum size effects. The features also make it possible to provide an improved efficiency for the optically functioning device fabricated using a quantum dot functional structure, in the transparent medium of which the ultra-fine particles are distributed homogeneously.
2. Description of the Prior Art
To employ semiconductor ultra-fine particles formed of Si families of IV materials for use in an optically functioning device that can emit light in the visible spectrum, it is indispensable to provide spherical ultra-fine particles which are controlled on the order of one nanometer in diameter. Moreover, the laser ablation method is preferably applied to the fabrication of the ultra-fine particles on the order of one nanometer in diameter.
For example, FIG. 1 is a conceptual view depicting an apparatus, disclosed in Japanese Patent Disclosure No. 9-275075, for applying the laser ablation method to a conventional target material to fabricate ultra-fine particles by deposition.
Referring to FIG. 1, a laser light beam is emitted from an excimer laser source 1 and travels through an optical system constituted by a slit 2, a condenser lens 3, a mirror 4, and a laser light inlet window 5 to be guided into a vacuum reaction chamber 6, where the laser light beam is focused on and thus radiates the surface of a target material 8 placed in a target holder 7, which is arranged inside the vacuum reaction chamber 6.
In addition, there is arranged a deposition substrate 9 in a direction normal to the surface of the target material 8. Substances detached or ejected from the target material 8 by laser ablation are captured or deposited on the deposition substrate 9.
An explanation will be given below in more detail to a case where semiconductor ultra-fine particles are fabricated with Si being employed as the target material in the apparatus configured as described above.
First, the vacuum reaction chamber 6 is pumped down to an ultra-high vacuum of pressure 1xc3x9710xe2x88x928 Torr by means of an ultra-high vacuum exhaust system 10, which is mainly constituted by a turbo-molecular pump, and then the ultra-high vacuum exhaust system 10 is closed.
Subsequently, a helium (He) gas is introduced through a rare-gas guide line 11 into the vacuum reaction chamber 6. The vacuum reaction chamber 6 is held at a constant pressure (of 1.0 to 20.0 Torr) with the low-pressure rare gas (He), the flow of which is controlled by means of a mass-flow controller 12 and which is differentially exhausted by means of a differential exhaust system 13 mainly consisting of a dry rotary pump. In the He gas atmosphere kept at a pressure of a few Torr, the surface of the target material is radiated with a laser light beam of a high-energy density (e.g., 1.0 J/cm2 or greater) to cause the substances to be detached or ejected from the target material.
The detached substance gives kinetic energy to the surrounding gas molecules, which are in turn urged to condense and grow in the gas atmosphere into ultra-fine particles of a few to a few tens of nanometers in diameter, the ultra-fine particles being deposited on the deposition substrate 9.
Originally, since the IV-group semiconductors are an indirect bandgap material, their bandgap transitions cannot be dispensed with phonons. The materials naturally cause much heat to be generated in their recombination, thus providing significantly decreased radiative recombination probability. However, the material shaped in ultra-fine particles having a diameter of a few nanometers causes the wave number selection rule to be relaxed in bandgap transitions and the oscillator strength to be increased. This in turn increases the probability of occurrence of radiative electron-hole pair recombination, thereby making it possible to provide intense light emission.
Here, the wavelength of emitted light (i.e., the energy of emitted photons) is controlled by making use of an increase in absorption edge emission energy (corresponding to bandgap Eg) provided by the quantum confinement effect resulted from a decrease in diameter of ultra-fine particles. FIG. 2 is an explanatory graph showing the correlation between the diameter of the aforementioned ultra-fine particles and the absorption edge emission energy thereof.
That is, to emit light at a single wavelength, it is indispensable to make the diameter of the ultra-fine particles uniform. If ultra-fine particles of a diameter corresponding to the emission wavelength can be generated and deposited within as narrow a diameter distribution as possible, it is made possible to fabricate an optically functioning device for emitting light of a single color.
As described in the aforementioned prior art, it is required to generate and deposit ultra-fine particles having a particle diameter distribution controlled to provide a single diameter of a few nanometers in order to fabricate an optically functioning device for emitting light at a single wavelength using semiconductor ultra-fine particles.
The prior art makes it possible to control the mean particle diameter by selecting as appropriate the pressure of an atmospheric rare gas or the distance between the target material and the deposition substrate. However, the prior art provides a still broad particle diameter distribution. Thus, it is difficult to obtain semiconductor ultra-fine particles of a uniform diameter distribution having, for example, a geometric standard deviation "sgr"g of 1.2 or less.
That is, this means that more aggressive particle diameter control is required. In addition, nm-sized ultra-fine particles are very sensitive to the contamination of impurities or defects due to their high surface atom ratio (e.g., about 40% at a particle diameter of 5 nm).
That is, it is required to provide a clean and damage-less process as a method for generating and depositing the particles. Moreover, adhering and depositing semiconductor ultra-fine particles directly onto a deposition substrate as in the prior art would tend to result in a thin film of a porous structure formed of a deposit of ultra-fine particles.
Suppose that electrodes are connected to such a porous structure to allow it to function as an optically functioning device. In this case, it may be required to optimize the structure somehow. On the other hand, in order to derive the quantum size effect originally provided for spherical ultra-fine particles to implement a new optical function representative of light emission, further optimized shape and structure may be required such as a structure having particles distributed homogeneously in a stable transparent medium.
In addition, since nm-sized ultra-fine particles have a very sensitive surface as described above, it may become necessary to form a quantum dot functional structure having the particles being homogeneously distributed in a stable transparent medium.
In addition, in order to obtain fine particles having a specified particle diameter, a fine particle classifier may be used for classifying the diameter of fine particles using the mobility which is dependent on the particle diameter. Such a fine particle classifier has been used for performance test of high-performance air filters for collecting and separating sub-micron fine particles with high efficiency, and for generating standard fine particles and measuring the particle diameter upon monitoring of cleaned atmosphere. The mobility employed for classifying the diameter of particles includes mainly the electrical mobility of charged particles in an electro-static field and the dynamic mobility caused by gravitational force. In addition, the aforementioned fine particle classifier has two main structures: a double cylinder and a disk type structure.
FIG. 3 is a schematic view illustrating the structure of a prior-art differential electrical mobility classifier, introduced in Japanese Journal of Aerosol Research Vol.2, No.2, p106 (1987) or in Japanese Journal of Powder Engineering Society Vol.21, No.12, p753 (1984). This differential electrical mobility classifier has a double cylindrical structure comprising an outer cylinder (of radius R1) 19 and an inner cylinder (of radius R2) 20 disposed inside the outer cylinder 19 concentrically with the outer cylinder 19. Referring to FIG. 3, charged fine particles 21 are transported by a carrier gas 22 to flow into the double cylinder classifier from the upper and portion thereof to be mixed with clean air or a sheath gas 23 flowing therein. The mixture gas of the charged fine particles 21 and the sheath gas 23 flows as a laminar flow over a length of L through the double cylinder portion. An electrostatic field is applied to this double cylinder portion with a direct current power supply 24 in a direction perpendicular to the flow of said mixture gas. This causes the charged fine particles 21 to draw an orbit in accordance with the electrical mobility of each particle. Since said electrical mobility is dependent on the diameter of fine particles, only those fine particles having a specific diameter arrive at a lower slit 25 and then are classified to be taken out of a carrier gas exhaust vent 26. The fine particles of other diameters are exhausted from a sheath gas exhaust vent 27 in conjunction with the sheath gas 23 or caused to move to and adhere to an inner collector electrode 28.
On the other hand, as a prior art fine particle classifier, a dynamic mobility classifier having a disk structure is disclosed in Japanese Patent Disclosure No. 9-269288. FIG. 4 is a schematic view illustrating the structure of the dynamic mobility classifier of the aforementioned disk type.
This disk-type dynamic mobility classifier comprises a disk-shaped upper disk 31, a disk-shaped lower disk 32 disposed opposite to and spaced apart by a predetermined distance from the upper disk 31, and a particle collector portion 33 attached to the lower disk 32 and disposed opposite to the upper disk 31. There is formed a cylindrical central suction duct 34, having an opening at one end thereof, on the central portion of the upper disk 31. There are formed a plurality of holes or slits 35 for introducing a carrier gas in the vicinity of the rim portion of the disk in the outward radial direction from the central suction duct 34. The lower disk 32 is provided with substantially the same diameter as that of the upper disk 31 and disposed generally in concentric relationship therewith. There are formed a plurality of holes or slits 36 for emitting a carrier gas on a portion apart by a predetermined distance in the outward radial direction from the center of the lower disk 32. The slits 35 provided on the upper disk 31 and the slits 36 provided on the lower disk 32 are a plurality of slits formed in an annular shape along a predetermined circumference, spaced apart at certain intervals. The distance radially outward from the center of the disk to the position of the slits 36 provided on the lower disk 32 is designed to be less than the distance radially outward from the center of the disk to the slits 35 provided on the upper disk 31. Between the upper disc 31 and the lower disk 32, there is defined a space or a classifying region 37. On the central portion of the particle collector portion 33, there is formed a cylindrical withdrawal duct 38 having an opening at one end thereof. The particle collector portion 33 is adapted to discharge classified fine particles from the withdrawal duct 38 in conjunction with the carrier gas.
Referring to FIG. 4, the classifying region 37 is formed in a space defined between the upper disk 31 and the lower disk 32, arranged to be concentric and parallel to each other. A sheath gas or an air flow 39 is introduced into the classifying region 37 from the periphery of the upper and lower disks 31, 32, being supplied from the outer rim inwardly in the radial direction. The air flow 39 takes place as a centripetal laminar flow through the classifying region 37 and is exhausted from the central suction duct 34 (indicated by arrow A1 in FIG. 4). Fine particles 40 are transported with a carrier gas 41 to be guided from the slits 35 provided on the upper disk 31 into the classifying region 37. The fine particles 40, which have been guided from the slits 35 provided on the upper disk 31 into the classifying region 37, are moved with the air flow 39 toward the center axis as well as drop from the upper disk 31 toward the lower disk 32 due to the gravitational field. Since the drop speed is dependent on the diameter of the fine particles 40, only those fine particles having a certain diameter are allowed to reach the slits 36 arranged on the lower disk 32, thus being classified and taken out of the withdrawal duct 38 (indicated by arrow A2 in FIG. 4). The particles having other diameters are exhausted from the central suction duct 34 in conjunction with the air flow or moved to the lower disk 32 to adhere to the surface thereof.
In the field of such fine particle classification technology, known is that the physical properties of ultra-fine particles having diameters from a few to a few tens of nanometers vary depending on the particle diameter. For example, the energy gap of semiconductor ultra-fine particles increases as the particle diameter decreases. Attempts have been made to create new devices by making use of the physical properties of the aforementioned semiconductor ultra-fine particles. As a substance for forming the aforementioned new device, Si has received attention. In this context, attempts have been made to create ultra-fine particles of Si having diameters from a few to a few tens of nanometers by making use of the pulse laser ablation in a rare gas. To create a new device employing the Si ultra-fine particles, it is necessary to classify the Si ultra-fine particles having various diameters on the order of a few to a few tens of nanometers and thus extract those Si ultra-fine particles having a narrow particle diameter distribution enough to regard the particles as having a single diameter. In addition, the mean particle diameter of the Si ultra-fine particles to be classified can be preferably varied.
On the other hand, the prior art disk type dynamic mobility classifier shown in FIG. 4 is adapted to classify fine particles having generally sub-micron diameters, employing the gravitational field for classifying the particle diameter. Since the gravitational field is constant, it is necessary to vary the flow rate of the air flow 39 in order to vary the mean particle diameter of the ultra-fine particles being classified. A variation in mean particle diameter of nm-sized ultra-fine particles requires a fine variation in flow rate of the aforementioned air flow 39. It is extremely difficult to control this fine variation in flow rate and to stabilize the flow rate.
Furthermore, in order to classify ultra-fine particles having sub-micron or less diameters without increasing the size of the aforementioned disk type dynamic mobility classifier (i.e., without increasing the projective distance of the annually formed slits 35, 36), it is necessary to apply a force greater in magnitude than the gravitational force to the ultra-fine particles in a direction perpendicular to a sheath gas flow or the air flow 39 (in the direction from the upper disk 31 to the lower disk 32) in the classifying region 37.
In addition, as a method for improving the classification resolution of the ultra-fine particles, such a technique is available that increases the classifying region from one stage to multiple stages to increase the number of times of classification. Referring to the double cylinder classifier shown in FIG. 3, for example, the double cylinder classifier described in the Japanese Journal of Powder Engineering Society Vol.21, No.12, p753 (1984) has the dimensions of the classifying region of L=400 mm, R2=15 mm, and R1=25 mm. In this context, suppose a cylindrical classifying region is further added to the outer periphery of the aforementioned double cylinder classifier to provide multiple stages of the classifying region. In this case, the overall dimensions of the classifier would be significantly increased. Therefore, it is necessary to employ a structure other than that of the double cylinder type to make the overall dimensions of the classifier small.
In order to solve the aforementioned prior art problems, the apparatus for fabricating a quantum dot functional structure according to the present invention is constructed as follows. That is, the apparatus comprises a fine particle generating chamber for generating ultra-fine particles by laser ablation and a fine particle classifying chamber for classifying the ultra-fine particles according to their particle diameters. The apparatus also comprises a transparent medium generating chamber for generating a transparent medium by laser ablation and a depositing chamber for depositing the ultra-fine particles onto a substrate and embedding the particles in the transparent medium at the same time.
With this apparatus, it is made possible to efficiently fabricate high-purity ultra-fine particles having a single particle diameter and uniform structure with their contamination and damage being alleviated. It is also made possible to deposit the particles onto the substrate in conjunction with the transparent medium and thus fabricate an optically functioning device employing, as an active layer, a quantum dot functional structure having the ultra-fine particles homogeneously distributed in the transparent medium.
As various embodiments according to the present invention configured as described above, the present invention provides an apparatus for fabricating a quantum dot functional structure characterized by being constructed as follows. That is, the apparatus comprises a fine particle generating chamber for generating fine particles and a fins particle classifying chamber for classifying fine particle generated in the fine particle generating chamber according to a desired particle diameter in a gas. The apparatus also comprises gas exhaust means for exhausting a ads for transporting the fine particles and transparent medium generating means for generating a transparent medium. The apparatus further comprises a depositing chamber for collecting fine particles classified in the fine particle classifying chamber onto a substrate as well as for collecting a transparent medium generated by the transparent medium generating means onto the substrate and for depositing the classified fine particles and the transparent medium onto the substrate. This makes it possible to efficiently fabricate high-purity ultra-fine particles having a single particle diameter and uniform structure with their contamination and damage being alleviated. It is also made possible to deposit the particles onto the substrate in conjunction with the transparent medium at the same time and thus fabricate an optically functioning device employing, as an active layer, a quantum dot functional structure having the ultra-fine particles homogeneously distributed in the transparent medium.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized by further comprising first transparent medium generating means arranged in the depositing chamber, and a second independent transparent medium generating chamber as the transparent medium generating means. For example, this can prevent fine particles of a material susceptible to oxidation from being oxidized when such a material is used as the transparent medium that can make the atmosphere near the deposition substrate oxidative upon generation of the transparent medium. Thus, the present invention can extend the range of selection of materials for fabricating a quantum dot functional structure.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized in that the fine particle generating chamber, the fine particle classifying chamber, and a transport path of fine particles in the depositing chamber are constructed on a straight line. Thus, the apparatus can prevent the exhaust conductance of a transport path from being reduced upon transportation of ultra-fine particles in a gas and the ultra-fine particles from being deposited in the transport path upon transportation of the particles, thereby leading to an improvement in yield of the particles.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized in that the pressure of the fine particle generating chamber and the pressure of the depositing chamber are controlled independently. Thus, the apparatus can control with accuracy the pressure for generating the ultra-fine particles and the transparent medium at the optimum value for each material, thereby making it possible to control with accuracy the structure and physical properties of the quantum dot functional structure.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized in that the gas exhaust means is controlled in accordance with the pressure of the depositing chamber. This makes it possible to provide a larger difference in pressure between the fine particle generating chamber and the depositing chamber, thereby improving the transport efficiency of the ultra-fine particles.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized in that the substrate in the depositing chamber is rotatable with respect to a direction of deposition of the fine particles and transparent medium. This makes it possible to improve the deposition efficiency when the ultra-fine particles and the transparent medium are alternately deposited.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized by further comprising a temperature control mechanism being capable of maintaining a transport path of fine particles at a constant temperature. This can prevent the ultra-fine particles from being deposited in the transport pipe by thermo-phoresis effect, also preventing the mutual cohesion of the ultra-fine particles.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized in that at least one of the fine particles and the transparent medium is generated using laser ablation, and a plasma plume produced upon generation is observed using a charge coupled device. The apparatus allows the laser ablation to be observed in real time, thereby making it possible to determine the stability of the laser ablation upon generation of the ultra-fine particles and transparent medium.
The present invention also provides an apparatus for fabricating a quantum dot functional structure, characterized in that at least one of the generated fine particles and the transparent medium is radiated with ultraviolet light to observe fluorescent light. The apparatus allows for observing in real time the process of generating the ultra-fine particles and the transparent medium, making it possible to efficiently capture the ultra-fine particles into the fine particle classifying chamber as well as efficiently deposit the transparent medium.
The present invention also provides a quantum dot functional structure fabricated by the aforementioned apparatus for fabricating a quantum dot functional structure. Thus, it is made possible to realize an optically functioning device employing an active layer with a structure having the ultra-fine particles homogeneously distributed in a stable transparent medium.
The present invention also provides am optically functioning device employing the aforementioned quantum dot functional structure as an active layer, thereby making it possible to improve the efficiency when compared with that of the prior art.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized by comprising the steps of generating fine particles; classifying the fine particles generated according to a desired particle diameter in a gas; exhausting a gas for transporting the fine particles after the classifying step; collecting the classified fine particles onto a substrate and generating a transparent medium at the same time; and depositing the classified fine particles and the transparent medium onto the substrate at the same time. The method makes it possible to efficiently fabricate high-purity ultra-fine particles having a single particle diameter and uniform structure with their contamination and damage being alleviated. It is also made possible to deposit the particles onto the substrate in conjunction with the transparent medium at the same time and thus fabricate an optically functioning device employing, as an active layer, a quantum dot functional structure having the ultra-fine particles homogeneously distributed in the transparent medium.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized in that the transparent medium is generated using, at the same time or alternately, any one of or both first transparent medium generating means, disposed in a depositing chamber for depositing the fine particles and the transparent medium, and second transparent medium generating means arranged independently. For example, this can prevent fine particles of a material susceptible to oxidation from being oxidized when such a material is used as the transparent medium that can make the atmosphere near the deposition substrate oxidative upon generation of the transparent medium. Thus, the present invention can extend the range of selection of materials for fabricating a quantum dot functional structure.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized in that the fine particles and the transparent medium are controlled independently of each other so that each pressure upon generation thereof becomes optimum at the same time, and thereby generated. Thus, the method can control with accuracy the pressure for generating the ultra-fine particles and the transparent medium at the optimum value for each material, thereby making it possible to control with accuracy the structure and physical properties of the quantum dot functional structure.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized in that the gas for transporting fine particles is exhausted, after the step of classifying the fine particles, in accordance with a pressure of the depositing chamber for depositing the fine particles and the transparent medium onto the substrate. This makes it possible to provide a larger difference in pressure between the fine particle generating chamber and the depositing chamber, thereby improving the transport efficiency of the ultra-fine particles.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized by further comprising the step of maintaining a path of the fine particles at a constant temperature after the step of classifying the fine particles. This can prevent the ultra-fine particles from being deposited in the transport pipe by thermo-phoresis effect, also preventing the mutual cohesion of the ultra-fine particles.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized by further comprising the step of observing, using a charge coupled device, a plasma plume produced when at least one of the fine particles and the transparent medium is generated using laser ablation. The method allows the laser ablation to be observed in real time, thereby making it possible to determine the stability of the laser ablation upon generation of the ultra-fine particles and transparent medium.
The present invention also provides a method for fabricating a quantum dot functional structure, characterized by further comprising the step of observing fluorescent light from the fine particles and the transparent medium, emitted when at least one of the fine particles and the transparent medium is radiated with ultraviolet light upon generation thereof. Thus, it is made possible to observe in real time the process of generating the ultra-fine particles, leading to an improved deposition efficiency.
The present invention also provides a quantum dot functional structure fabricated by the aforementioned method for fabricating a quantum dot functional structure. Thus, it is made possible to realize an optically functioning device employing an active layer with a structure having the ultra-fine particles homogeneously distributed in a stable transparent medium.
The present invention also provides an optically functioning device employing the aforementioned quantum dot functional structure as an active layer, thereby making it possible to improve the efficiency when compared with that of the prior art.
As described above, according to the present invention, nm-sized high-purity ultra-fine particles having a single diameter and uniform structure can be fabricated efficiently with their contamination and damage being alleviated and deposited on a deposition substrate. In addition, it is also made possible to fabricate an optically functioning device which employs as the active layer the quantum dot functional structure having the ultra-fine particles distributed homogeneously in the stable transparent medium.
Furthermore, as an improvement of a disk type dynamic mobility classifier that can be incorporated into the apparatus for fabricating a quantum dot functional structure, the present invention allows a direct current voltage to be applied between the upper and lower disks. This makes it possible to establish an electrostatic field in the vertical direction in a classifying region (in a direction perpendicular to an air flow). Thus, when fine particles are charged which are introduced into the aforementioned disk type dynamic mobility classifier, it is made possible to classify the charged fine particles not according to the dynamic mobility caused by the gravitational field but according to the electrical mobility caused by the electrostatic field. Increasing the direct current voltage applied between the upper and lower disks makes it possible to produce an electrostatic force that is greater than the gravitational force. This makes it possible to classify nm-sized ultra-fine particles without increasing the aforementioned disk type dynamic mobility classifier in size (without increasing the annular guide slits and the projection distance of the annular slits).
Furthermore, varying the direct current voltage applied between the upper and lower disks makes it possible to change the strength of the electrostatic field with accuracy. Thus, this also makes it possible to vary the mean particle diameter upon classification of nm-sized charged ultra-fine particles at a constant flow rate of air.
Furthermore, in the aforementioned disk type dynamic mobility classifier, the classifying region is constituted by multiple stages instead of one stage. This makes it possible to reduce the size of the overall classifier and improve the classifying resolution of nm-sized charged ultra-fine particles. More specifically, a third disk is arranged on the lower portion of the lower disk of the aforementioned disk type dynamic mobility classifier, concentrically in parallel thereto. A space between the lower disk and the third disk is employed as a second stage classifying region. In the same manner as this, a fourth disk, a fifth disk, . . . are arranged to define a third stage classifying region, a fourth stage classifying region, . . . .
As described above, according to the present invention, a disk type dynamic mobility classifier is implemented in which a direct current voltage can be applied between upper and lower disks and which is provided with a multi-stage classifying region, thereby providing a disk type ultra-fine particle classifier for classifying nm-sized ultra-fine particles with good resolution.
Accordingly, it is a first object of the present invention to provide an apparatus for fabricating a quantum dot functional structure, in which nm-sized high-purity ultra-fine particles having a single diameter and uniform structure can be fabricated efficiently with their contamination and damage being alleviated and deposited on a deposition substrate. In addition, the apparatus makes it possible to fabricate an optically functioning device which employs as the active layer the quantum dot functional structure having the ultra-fine particles distributed homogeneously in the stable transparent medium.
In addition, it is a second object of the present invention to provide an ultra-fine particle classifier which can classify nm-sized ultra-fine particles with good resolution.